Friday, November 6, 2009

A new Sibley Guides blog and website!

This blog has been moved to a new and improved blog and website at

Thanks for your support!

Comments here are closed. All future posts will be at the new site, and all of the old posts and comments from this blog have been transferred there as well.

Tuesday, October 20, 2009

Ageing Canada Geese

Determining the age of a goose can be helpful for identification, and can also reveal some interesting behavioral and ecological information by enabling you to sort out family groups and subflocks. Given good views it's not hard to distinguish Canada Geese that are still in juvenal plumage from those that are in adult plumage. By October many immatures (hatched in the summer a few months earlier) will have molted most of their body feathers into adult-like plumage, while others have not yet begun to molt. In general, consistent with other species of birds, the late-molting juveniles are long-distance migrants from far northern breeding grounds, while local Lower-48 resident Canada Geese molt in early fall.

Juvenile (top) and adult (bottom) Canada Goose. 19 Oct 2009. Concord, MA. Photos copyright David Sibley.

Differences apparent in these photos are:

  • Juvenile overall has lower contrast markings, especially blurry and obscure pale edges on back feathers
  • Juvenile has all feather tips narrow and rounded, vs adult feather tips broader and more square or flat-tipped. This creates a strongly barred pattern on the adult with pale lines straight across the back and well-marked pale and dark bars on the flanks.
  • Juvenile has neck not quite as dark blackish as the adult, and the dark neck blends into the paler breast color; vs the adult has a truly black neck (in shadow in the lower photo here) with very distinct edge and no blending to pale breast (but in some dark western subspecies the black neck may blend into a dark brown breast even on adults)
Similar differences in feather shape and pattern should allow ageing of other species of geese as well.

Monday, October 19, 2009

Obsolete Bird Names

Have you seen any Field Plovers lately? Blue Nutcrackers? Day Owls? These are all alternate common names of North American birds. Richard Banks has assembled a complete list of outdated English names of North American birds along with the current official names for those species, and it makes a great resource to find out what early authors meant, as well as just fun browsing. You can read it at Obsolete English Names of Birds and their Modern Equivalents

If only there was a similar resource for North American plants.

Wednesday, October 14, 2009

Corrections to The Sibley Guide to Trees

updated 16 Oct 2009

Here are page-by-page corrections and changes for the Sibley Guide to Trees. This listing will be updated periodically as issues come to my attention. Please feel free to leave comments or send me an email if you notice anything that is not listed here.

p 3 - California Torreya, the common and scientific names should be justified left
p 28 - Red Pine needs a new name to distinguish it from Japanese Red Pine. I propose American Red Pine.
p 146 - Black Hickory caption should say "husk very similar to Pignut..."; Sand Hickory, captions should read "fruit similar to Pignut..." and "twigs similar to Pignut..."
p 179 - Tanoak - caption at bottom right should specify “pale underleaf conspicuous among dense, dark foliage”
p 189 - Southern Red Oak leafy twig, the caption saying “acorn identical to Southern Red Oak” should instead say “acorn identical to Cherrybark Oak”
p 227 - Tung-Oil Tree - leaves should be larger
p 252 - Glossy False-buckthorn includes the three leaves at the top of the column and the map at the bottom. The yellow box of text and the three leaves immediately below that are Ceanothus. The map should be moved up above the box text to make clear that it represents the range of Glossy False-buckthorn.
p255 - Rose Family Intro, near bottom of column 1 should say "Plums and Cherries (Prunus) page 256" [not page 254]
p 302 - Slippery Elm, in the third sentence "fragrant" is misspelled.
page 332 - Intro to maples, the third paragraph begins "All maples have palmately compound leaves..." This should instead say "Nearly all maples have palmately lobed leaves..." And could go on to elaborate that a few species have the leaves so deeply lobed that they are compound, and Boxelder is unique among the maples in having pinnately compound leaves.
p 345 - Mountain Maple underleaf should be rotated with stem down, (same for Vine Maple)
p 392 - First line of text - capitalize Paulownia family
p 393 - Aralia Family, add a sentence saying "Castor-Aralia (p 322) is also a member of this family and should be placed here." [and in a future edition it will be]

Pignut p 145
Rose Family p 254
And it is suggested (see comments below) that species like Northern White-Cedar and Western Redcedar should have an entry in the index under "cedar". [and presumably there are other examples like this]

Saturday, October 3, 2009

Trees with opposite leaves

In the past, tree identification guides have emphasized the presence of opposite leaves as one of the most important field marks. In the Sibley Guide to Trees I used a more holistic approach, like modern bird identification, giving equal weight to all parts of the tree. A tree might catch your attention because of its flowers, twigs, bark, fruit, etc., and I don’t think it’s helpful if the field guide then asks you to check whether the leaves are alternate or opposite. It’s better to get to know all of the distinguishing characteristics of oaks and maples, for example, so that you can recognize them by multiple features, rather than having to check leaf arrangement.

In some cases, however, when you encounter a confusing tree, the presence of opposite leaves can be a useful pointer to help narrow the range of possibilities. So below I’ve put together a list of trees with opposite leaves. Interestingly, almost all of these species show up in the last 80 pages of the guide.

Definition from p xxiv of The Sibley Guide to Trees:
Leaves that grow singly from the twig, without another leaf directly across the twig, are called alternate, as they usually grow from alternate sides of the twig. Leaves that grow in pairs from opposite sides of the twig at the same level are called opposite. If three or more leaves all grow from the same level on the twig they are called whorled.
These patterns can be detected even more easily in winter, as buds and leaf scars on the twigs match the arrangement of the leaves (either opposite or alternate), and branching tends to be opposite or alternate as well. Also note that twigs with alternate leaves tend to grow in a zigzag pattern, while trees with opposite leaves generally have straighter twigs.

Species counts only include those illustrated in the guide.

North American Trees with Opposite Leaves

  • Katsura, Genus Cercidophyllum (p. 101) – one introduced species [the only species in its family]
  • Eucalyptus, Genus Eucalyptus (p 109) – three introduced species have juvenile foliage opposite, but adult foliage alternate [Crape-myrtle, in the same family, has alternate leaves]
  • Paper Mulberry, Genus Brousonettia (p. 248) – one introduced species [other genera in the Mulberry family do not have opposite leaves]
  • Lyontree, Genus Lyonothamnus (p. 293) – one native species [other genera in the Rose family do not have opposite leaves]
  • Beebee Tree, Genus Evodia (p. 321) – one introduced species [related to Corktree, but other species in the Rue family do not have opposite leaves]
  • Corktree, Genus Phellodendron (p. 323) – one introduced species [related to Beebee Tree, but other species in the Rue family do not have opposite leaves]
  • Buckeyes, Genus Aesculus (p. 326-331) – six native and one introduced species [related to maples, but other genera in the Soapberry family do not have opposite leaves]
  • Maples, Genus Acer (p. 332-351) – 13 native and ten introduced species [related to buckeyes, but other genera in the Soapberry family do not have opposite leaves]
  • Dogwoods, Genus Cornus (p. 355-358) – three (out of four) native and one introduced species have opposite leaves [Dove Tree, in the same family, does not]
  • Buttonbush, Genus Cephalanthus (p. 374) – one native species [the only North American tree in the Madder family]
  • Ash, Genus Fraxinus (p. 376-385) – 17 native and four introduced species [all other genera in the Olive family also have opposite leaves]
  • Japanese Tree-lilac, Genus Syringa (p. 386) – one introduced species [all other genera in the Olive family also have opposite leaves]
  • American Fringetree, Genus Chionanthus (p. 387) – one native species [all other genera in the Olive family also have opposite leaves]
  • Common Olive, Genus Olea (p. 388) – one introduced species [all other genera in the Olive family also have opposite leaves]
  • Devilwood, Genus Osmanthus (p. 388) – one native species [all other genera in the Olive family also have opposite leaves]
  • Desert-willow, Genus Chilopsis (p. 389) – one native species with opposite or whorled leaves [related to Catalpas, other genera in the Trumpet Creeper family also have opposite leaves]
  • Catalpas, Genus Catalpa (p. 390-391) – two native and one introduced species have opposite or whorled leaves [related to Desert-willow, other genera in the Trumpet Creeper family also have opposite leaves]
  • Royal Paulownia, Genus Paulownia (p. 392) – one introduced species [the only species in the family]
  • Elders, Genus Sambucus (p. 397) – many native species mostly shrubs [related to Viburnums, all other genera in the Muskroot family have opposite leaves]
  • Viburnum, Genus Viburnum (p. 397) – many native species mostly shrubs [related to Elders, all other genera in the Muskroot family have opposite leaves]

Thursday, October 1, 2009

Probability in bird identification

One of my regular birding spots is a small farm field near my house in Concord, Massachusetts. I can walk around the entire place in about 20 minutes, but I usually take about two hours and get in some good sparrow-study. The clump of taller vegetation shown here is always a good spot to study birds that come up out of the grass and weeds and perch a little more conspicuously, so I usually spend a lot of time standing here looking at sparrows through my telescope.

On one visit in September 2008, I noticed that the three common species of sparrows – Savannah, Song, and Swamp – were using different parts of the vegetation, and I made a note to watch for this on future visits. It seems to be fairly consistent, so I can now produce a kind of "map" of sparrow-regions within the shrub thicket.

I've divided the thicket into three zones that I'll call the margins, the middle, and the depths. I was seeing mainly Savannah, Song, and Swamp Sparrows, and those are the species I'll discuss here.
  • The Margins are occupied by Savannah Sparrows and are rarely used by the other two species (almost never by Swamp)
  • The Middle zone was used by all three species, although I would say it was preferred by Song Sparrow, and was used rarely by Swamp Sparrow.
  • The Depths were the home of Swamp Sparrows, which rarely venture into the other zones. Song and Savannah sparrows would also go deep into the shrubs when alarmed (e.g. if a hawk flew by), but their typical behavior when flushed by me was to land on the outer edges of the thicket.
Of course, in real life this thicket is three-dimensional, but the zones I've drawn are more like a cross-section. So a Savannah Sparrow could head for the lower parts of the thicket and still land in the margins of that shrub.

This may seem trivial, but it's really no different from habitat and other "soft" identification clues. You certainly can't identify a bird solely by its chosen perch, but we should never doubt the capacity of the human mind to discern subtle differences in probability (just look at baseball batting averages). The more I watched the more I realized that these are helpful clues to identification, just as date and habitat are helpful clues. A sparrow that flies right up into the highest twigs of the cherry sapling is almost certainly a Savannah, and it doesn't take much more to confirm that. A sparrow that goes straight into the deep cover and then barely pokes its head out is likely to be Swamp (or White-throated, or sometimes Song, but at least we can start with some idea of the likely species).

Check it out in your local area. I'd be interested to hear how it works in other places and with other species. And of course the same principle should be applicable to lots of other situations.

Wednesday, September 23, 2009

Correction to maples, page 332

Reader Sherman Dunnam sent me a note about an error in the introduction to maples on page 332. The third paragraph begins "All maples have palmately compound leaves..."

This should instead say "Nearly all maples have palmately lobed leaves..." And could go on to elaborate that a few species have the leaves so deeply lobed that they are compound, and Boxelder is unique among the maples in having pinnately compound leaves.

Updates to the Tree Guide

I have spent the last seven years working intensively on my new Guide to Trees, but of course a few misstatements and errors somehow managed to creep in. I'm always interested in learning more, correcting my mistakes, and passing along to you - the reader - newer, better, more accurate information.

So if you notice any errors, or anything that seems to be missing or misleading, please don't hesitate to let me know, either by leaving a comment here or sending me an email. I'll be posting additions and corrections as time allows. (And the same goes for any of my bird guides, of course)


Tuesday, September 22, 2009

Tree Guide news

Two events related to The Sibley Guide to Trees: It is the featured product today on the Daily Grommet website. I’ll be checking in periodically, responding to comments and answering questions through noon tomorrow (Wednesday Sep 23rd) .

And “The Art of Identification” - a show of my artwork and field sketches at the Massachusetts Audubon Society’s Visual Arts Center in Canton, MA, will be opening this coming Sunday Sep 27th, and showing until January 2010. More details are here.

Thursday, September 10, 2009

My latest project

After seven years of work, my new Guide to Trees goes on sale officially on September 15th (but those who live near Philadelphia can get it as early as Saturday the 12th when I'm there for book-signings). I'll be traveling quite a bit this fall to book-signings across the US and Canada, doing a variety of publicity-related things, and adding tree identification to the topics I cover here on this blog. Coming soon...

More information about the book can be found at the Knopf website, and a list of my confirmed book-signing events for the fall is here.

Thursday, July 9, 2009

More vocal copying by American Goldfinch, Purple Finch, and Ovenbird

After listening carefully to Pine Siskins this year to pick out the calls of other species (see my earlier post) I have run across several other mimics. In early June in Arlington, Massachusetts I heard a remarkable American Goldfinch song that incorporated several phrases of a Song Sparrow song along with Northern Flicker and I suspect other species were being copied but I couldn't pick them out. This is the first time I've heard an American Goldfinch that included copies of other species in its song, and these were very clear and obvious copies, but as Dan Lane pointed out in comments on my previous Siskin post American Goldfinch seems to be exceptional – the one American member of the genus that does not copy. Apparently they do, at least occasionally.

In western Massachusetts in late June 2009 I heard a male Purple Finch giving its territorial song. This territorial song began with a bright warble like a typical Purple Finch song, then continued into a longer and more varied section – a jumble of short song phrases and copies of other species. I was able to pick out copies of American Robin, Tufted Titmouse, Eastern Towhee, and Hairy Woodpecker, and again I suspect there were more. In this case the copying has been previously reported in the literature, with the BNA account mentioning Barn Swallow, American Goldfinch, Eastern Towhee, and Brown-headed Cowbird.

Several years ago I noticed a few copies in the flight song of an Ovenbird, and I've heard a few others since then. Over several days in late June 2009 I was able to hear many flight songs from one Ovenbird in Colton, New York, which included copies of Red-eyed Vireo song phrase, Robin tut note, and a Purple Finch song phrase. There are very few recordings of the flight song (since it is given at such long intervals) but one on the Stokes Guide to Bird Songs: Eastern includes a lot of mimicry. Listening carefully to that I can hear a snippet of Winter Wren song, the trill of Eastern Towhee song, Red-eyed Vireo song phrase, and ending with White-eyed Vireo song. It's such a jumble of calls that (like the Siskin song) it's very hard to pull out all the copies, so I suspect there are others, and this whole performance – other than the few typical Ovenbird song phrases in the middle – might be copies. This is another instance of vocal copying that has never been reported before, as far as I know. The BNA account for Ovenbird makes no mention of it.

It's interesting that both the Purple Finch and Ovenbird use copies in these sort of extended or auxiliary songs, and not in the more frequently heard "typical" songs.

I should point out a very interesting new blog by Nathan Pieplow about bird vocalizations, with a comment on the Pine Siskin issue
here, and lots of other good stuff. And another blog about bird voices by Paul Driver, with a recent post about Mimicry here.


McGraw, Kevin J. and Alex L. Middleton. 2009. American Goldfinch (Carduelis tristis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

Van Horn, M. A. and T.M. Donovan. 1994. Ovenbird (Seiurus aurocapilla), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

Wootton, J. Timothy. 1996. Purple Finch (Carpodacus purpureus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

Wednesday, June 24, 2009

Promising new window film may prevent bird collisions

The most recent research by Dr. Daniel Klem, who has been studying the problem of bird/window collisions since the 1990s, has just been published in the June 2009 Wilson Bulletin. It is available, along with lots of other bird/window resources, at his website here.

Klem tested the effectiveness of various window treatments confirming, for example, that single decals on a window are not effective at preventing bird collisions, and that the non-reflective Collidescape film is nearly 100% effective at preventing collisions. Several different UV-absorbing films were tested with small to moderate success.

Beginning in 2007 Klem tested a new film developed in collaboration with Dr. Tony Port, a chemist at CPFilms. This film has vertical strips that reflect UV wavelengths alternating with vertical stripes that absorb UV wavelengths. The UV reflection and the alternating pattern seems to be very effective at deterring birds – only a few birds collided with the windows treated with this film, and none of those collisions were fatal. The real promise of this film lies in the fact that it appears clear to humans, and should be relatively easy to apply to existing windows!

Hopefully the company, CPFilms, can be convinced that there is a market for this window film, and start producing it commercially. I can think of dozens of windows that I would put it on.

Klem, Daniel, Jr. 2009. Preventing Bird-Window Collisions. The Wilson Journal of Ornithology 121:314 –321

Thursday, May 14, 2009

Identification of Belding's Savannah Sparrow

In early March 2009 it was my pleasure to spend a few days in California at the San Diego Bird Festival. On my first morning in San Diego I headed straight out to the Tijuana River National Estuarine Research Reserve hoping to spend a few hours studying Savannah Sparrows (who wouldn't, right?). I had seen some Belding's Savannah Sparrows in March 2008 in Orange County, California, and realized that the treatment of this endangered subspecies in my field guide was not very good, so I was anxious to learn more.

I succeeded in seeing dozens of the resident Belding's, with many birds singing and territorial around the marsh. I also saw quite a few of the migratory northern subspecies of Savannah Sparrows (ranging from grayish to brownish) in weedy areas around the margins of the marsh.

The image below compares two of my photos showing the most obvious differences between the subspecies. Compared to northern birds, Belding's has:
  • longer and thicker bill with curved culmen
  • darker and much thicker streaks on the underside
  • darker streaks on the upperside and coarser and darker markings on the face
  • Belding's never (as far as I could see) raises a crest, while the northern birds often look slightly crested and show a distinct peak on the rear crown
Belding's (left) and Northern-type Savannah Sparrows. Photos 5 and 8 March 2009, San Diego, CA 
 copyright David Sibley (digiscoped with Nikon Coolpix 4300 handheld to Swarovski telescope)

Differences in behavior, habitat, and molt were also fairly obvious, and should be very helpful for identification (but bill and plumage features should be checked to confirm). 
  • Belding's in early March were singing and territorial, chasing each other across the marsh, while the northern birds were in small, loose flocks moving together widely across dry grassy and weedy areas around the marsh
  • Belding's flight was low and weak, with round body and fluttering wingbeats, reminding me of Sharp-tailed Sparrow, unlike the stronger, higher, more "bounding" and undulating flight of northern Savannahs
  • in early March the Belding's were all in clean fresh plumage, while the northern birds were molting and scruffy with missing feathers all over the head and body
I could not hear any difference in the song of Belding's compared to Savannah Sparrows from farther north.

I was struck by how much stockier and larger-billed the Belding's looked in comparison to the northern subspecies, in many ways intermediate between northern and "Large-billed" Savannah Sparrows. Apparently some populations intermediate between Belding's and northern birds are resident farther north along the California coast, but alongside the migratory northern subspecies the Belding's Savannah Sparrows in southern California seem quite distinctive.

Wednesday, May 13, 2009

Vocal copying by Pine Siskins

[10 July 2009 - update added at end]

In July 1990 I spent several weeks birding in west-central British Columbia, where Pine Siskins were one of the most conspicuous birds, and I became convinced that Pine Siskins were copying the sounds of other species of birds and incorporating these sounds into their songs. I was able to identify American Robin (squeal call), Evening Grosbeak (“krrr” call), Warbling Vireo (scold call and a song phrase), White-winged Crossbill (flight call and trill), and even House Sparrow (calls).

Since then I’ve heard this in many other places, and come to expect it whenever I hear siskins singing. From late March to early April 2009 I had the opportunity to listen to several individuals repeatedly in my yard in Concord, Massachusetts, and I was able to identify the following 14 species in the siskins’ songs:

Evening Grosbeak “krrr” call

American Robin, flight squeal and single “pup” call

Eastern Bluebird, rattling flight call

Eastern Towhee, “chewink”

Black-capped Chickadee, a single “dee” note

Northern Flicker, “kew” call

Hairy or other woodpecker, “wika” call

Dark-eyed Junco, soft “tew-tew-tew” call

Eastern Phoebe, song phrase and chase call

Blue-headed Vireo, two syllables of scolding call

Red-eyed Vireo, scold call

Song Sparrow, “jimp” call

Ruby-crowned Kinglet, two-syllabled whistled song phrase

Common Redpoll, flight call

These sounds can be very challenging to pick out, since the song moves along so rapidly that the bird is already singing other sounds while your brain struggles to process and identify the previous sound. The Evening Grosbeak call is commonly used and distinctive enough to stand out each time, but other calls like the Eastern Bluebird rattle simply blend in with the staccato jumble of sounds.

In my yard the same sounds were used repeatedly day after day, so as I became familiar with them I was better able to pick them out, and started to notice other copied sounds. I could not distinguish individual siskins, so I don’t know how many sounds were shared among birds, but the fact that the Evening Grosbeak call and American Robin call were heard virtually every time I listened carefully (and were heard years earlier in BC as well) suggests that most or all siskins use these calls, and probably share a lot of other sounds as well.

If you’d like to hear some of this, check out the recording catalog number 133352 at Cornell's Macauley Library, July 2, 2007, recorded in Newfoundland by Geoffrey Keller. [You’ll have to Search audio for Pine Siskin, and then find this recording]

The bird starts singing at about the 40 second mark, and between 47 and 56 seconds imitates Common Redpoll, Evening Grosbeak, Hairy Woodpecker, Dark-eyed Junco, American Tree Sparrow and American Robin (and probably others). Notice the clear sing-song notes of American Tree Sparrow at about 55 seconds, with the abrupt “pup” of American Robin following immediately after.

Amazingly, this vocal copying seems to have been almost completely overlooked. In Bent’s life histories (online here) copying of Evening Grosbeak and (in a captive bird) Common Redpoll and Canary, are mentioned. A study of vocal copying by Lawrence’s and Lesser goldfinches (Remsen et al, 1982, pdf here) specifically says that the song of Pine Siskin does not include any copied sounds, and the Birds of North America account makes no mention of it.

Presumably the sounds that are learned are simply the ones that are common in the siskins’ environment, so siskins from the western mountains must learn some sounds that are not heard in the east, and vice versa. Another Macauley Library recording from California (catalog number 120288) includes copies of Lesser Goldfinch flight calls, among others. Given how much siskins wander, it would make a really interesting exercise to listen for copied sounds that are “out-of-range”. Hearing, for example, Eastern Towhee in British Columbia or Lesser Goldfinch in Massachusetts would give you some idea of how far that individual siskin has traveled.

Update - in early June 2009 I spent a week at The Nature Conservancy's Pine Butte Guest Ranch near Choteau Montana, and was able to hear several Pine Siskins singing. I was able to hear copies of the vocalizations of seven species there. In addition to the now-expected calls of American Robin, Dark-eyed Junco, Common Redpoll, and Evening Grosbeak, the Montana siskins copied Western Tanager call, Mountain Chickadee dee note, and a Cassin's Vireo song phrase. Clearly these birds had learned their calls in the west, and would stand out in the east. So the challenge remains – to find a siskin that has traveled cross the continent, and recognize it based on the sounds it has learned.